The present invention refers to a DC-DC converter usable as a battery charger and to a method for charging a battery.
For charging batteries, for example batteries of cell phones, the use of DC-DC converters operating as battery chargers and able to perform various charging algorithms for NiCd, NiMH and TiIon batteres is known.
FIG. 1 illustrates a known step-down DC-DC converter usable as a battery charger.
The DC-DC converter, indicated as a whole by the reference number 1, comprises a switch 2, for example formed of a MOS transistor, the opening and closing whereof is controlled by a driving stage 4, and presenting a first terminal connected to a supply line 6 biased at the voltage VCC and a second terminal connected, via a diode 8, to ground; an inductor 10 and a sense resistor 12 series-connected between the second terminal of the switch 2 and a node 14, which is in turn connected, via a diode 16, to a positive pole of the battery 18 to be charged, which presents its negative pole connected to ground; a capacitor 20 connected between the node 14 and ground; and a voltage divider 22, formed of two resistors 24, 26, connected in parallel to the battery 18, and presenting an intermediate node 28 on which it supplies a voltage VFB proportional, through the division ratio, to the voltage VBAT present between the poles of the battery.
The DC-DC converter 1 further comprises a filtering stage 30, typically including an operational amplifier, presenting a first input and a second input terminals connected across the sense resistor 12, and an output terminal supplying the filtered voltage VFR present across the sense resistor 12; a differential current error amplifier 32 presenting an inverting terminal connected to the output terminal of the filtering stage 30, a non-inverting terminal receiving a reference voltage VR, and an output terminal connected to an output node 34 through a decoupling diode 36 presenting the anode terminal connected to the output node 34 and the cathode terminal connected to the output terminal of the current error amplifier 32; and a differential voltage error amplifier 42 presenting an inverting terminal connected to the intermediate node 28 of the voltage divider 22 and receiving from the latter the voltage VFB, a non-inverting terminal receiving a reference voltage VREF, and an output terminal connected directly to the output node 34.
In particular, the battery charsing current IBAT depends upon the reference voltage VR, which is generated by causing a constant current supplied by a current generator 40 connected in series to a resistor 37, to flow in the resistor 37 itself, the voltage present across the resistor 37 is then taken.
The current error amplifier 32 and the voltage error amplifier 42 are moreover biased through respective bias current generators 44, 46 supplying, respectively, a bias current IP and a bias current IV, both of which arc constant.
Finally, the DC-DC converter 1 comprises a zero-pole compensation network 48 including a resistor 50 and a capacitor 52 series-connected between the output node 34 and ground; and a differential comparator 54 known as PWM (Pulse Width Modulator) comparator, presenting an inverting terminal receiving a comparison voltage VC which has a sawtooth waveform, a non-inverting terminal connected to the output node 34, and an output terminal connected to the input of the driving stage 4 of the switch 2, basically operating as pulse width modulator and supplying at an output a voltage having a square waveform, the duty cycle whereof is a function of the voltage present on the output node 34 itself.
The operation of the DC-DC converter 1 is known and will here be referred to solely as regards the aspects necessary for understanding the problems lying at the basis of the present invention.
In particular, during the battery charging phase, the current error amplifier 32 prevails over the voltage error amplifier 42, and the DC-DC converter 1 operates in current regulation mode, behaving as a constant current generator.
During the current regulation phase, the battery charging current IBAT causes a voltage drop across the sense resistor 12, and this voltage, filtered by the filtering stage 30 so as to obtain the mean value thereof, is supplied to the current error amplifier 32, which operates to regulate this voltage so that it may assume a value equal to that of the reference voltage VR present on its own non-inverting terminal.
In parallel to the current error amplifier 32 there operates the voltage error amplifier 42, and in particular the current error amplifier 32 prevails over the voltage error amplifier 42 as long as the voltage VFB is lower than the reference voltage VREF, i.e., as long as the differential input voltage xcex94V=VREFxe2x88x92VFB present between its input terminals is negative, thus determining the unbalancing of the voltage error amplifier 42.
In detail, the current error amplifier 32 and the voltage error amplifier 42 are designed so that, during the current regulation phase, the diode 36 is on, and the current error amplifier 32 controls, through the comparator 54, the duty cycle of the signal issued by the comparator 54 so as to render the voltages present on its inverting and non-inverting terminals equal.
The current error amplifier 32 performs a negative feedback. In fact, a possible variation in the battery charging current IBAT results in an unbalancing of the current error amplifier 32, with consequent variation in the voltage of the output node 34, and hence of the duty cycle of the output signal of the comparator 54, which acts to restore the programmed value of the battery charging current IBAT.
During the current regulation phase, the battery 18 is thus charged with a constant current according to the value programmed via the current generator 40 and the resistor 36, and the battery voltage VBAT increases progressively towards the full charge value.
In the vicinity of this full charge value, the battery charging current IBAT starts decreasing until it zeroes, after which the DC-DC converter 1 enters the voltage regulation phase in which the voltage error amplifier 42 prevails over the current error amplifier 32 and controls the battery voltage.
In particular, during transition from the current regulation phase to the voltage regulation phase, the voltage error amplifier 42 is balanced, the voltage of the output node 34 decreases progressively until the diode 36 is off, and the battery charging current IBAT decreases, thus unbalancing the current error amplifier 32.
One drawback of the DC-DC converter 1 described above lies in the circuit topology which causes the evolution of its operation from the current regulation phase to the voltage regulation phase to depend to a large extent upon the transcharacteristic of the differential input stage of the voltage error amplifier, a dependence which results in the DC-DC converter 1 not being able to supply a battery charging current IBAT that is constant up until the battery full charge voltage is reached.
The disclosed embodiments of the present invention provide a DC-DC converter usable as a battery charger, which is able to supply a battery charging current that is constant up until the battery full charge voltage is reached.
A further aspect of the disclosed embodiments of the present invention is providing a method for charging a battery that makes it possible to supply to the battery a charging current that is constant up until the battery full charge voltage is reached.
In accordance with the disclosed embodiments of the invention, a DC-DC converter usable as a battery charger is provided, including a current error amplifier means and voltage error amplifier means connected in parallel to control the charging phase of a battery, and a gradual turning off means gradually turning off the current error amplifier means in a battery charging end phase. Ideally, the gradually turning off means comprises first current generating means supplying a first bias current for the current error amplifier means and configured to decrease an amplitude in the battery charging end phase. Thus, the first bias current presents a substantially constant amplitude during the battery charging phase preceding the end phase.
In accordance with a method of the present invention disclosed herein, charging of a battery includes supplying a current to the battery using a DC-DC converter comprising current error amplifier means and voltage error amplifier means connected in parallel to control charging of the battery and gradually turning off the current error amplifier means in a battery charging end phase. Ideally, the gradually turning off step comprises supplying to the current error amplifier means a first bias current having a decreasing amplitude in the battery charging end phase. Thus, the first bias current presents a substantially constant amplitude during a battery charging phase preceding the end phase.